Windmills and wind turbines are well known in the prior art. Such mills typically include a number of blades or vanes connected to a rotatable shaft. Wind flowing across the mill will cause hydrodynamic forces (i.e., forces generated by the movement of fluids including air and water) to build upon the blades, thereby causing the shaft and blades to revolve about the central axis of the shaft.
In order to increase efficiency of such known mills, and thus generate more power, mills have been constructed such that the blades are adjustable to present a variable surface area to the hydrodynamic flow of wind. When an individual blade is in the part of its revolution where the blade is moving parallel to the flow of the hydrodynamic medium, the blade adjusts to present maximum surface area to the medium. Conversely, when the blade is in the part of its revolution where the blade is moving perpendicular to the flow of the hydrodynamic medium, the blade adjusts to present minimum surface area to the medium. In this regard, the mills of the prior art typically include blades having two symmetrical halves. Minimum surface area is presented to the hydrodynamic medium by opening the two blade halves to a position parallel to the flow of the medium. In other words, the blade halves are oriented to a horizontal position to present minimum surface area to the hydrodynamic medium. Maximum surface area is presented to the hydrodynamic medium by rotating the trailing edges of each blade half away from one another so that each blade half forms a substantial angle relative to the medium such that a maximum surface area is presented to the hydrodynamic medium. To present maximum surface areas, the blade is rotated to a substantial angle relative to the hydrodynamic medium. Similar arrangements having single rotatable blades are also known.
While these known devices attempt to increase efficiency of the mill, these devices suffer from the same disadvantages. Specifically, the rotating symmetrical blade halves become unstable during rotation, causing the hydrodynamic medium to escape around the blades, which decreases efficiency. Further, the blades of the known devices require some mechanical advantage or mechanism to orient the blade halves away from one another. However, when the blades open cooperatively to define a broad profile and present a maximum surface area to the hydrodynamic medium, the blades no longer appear to operate as a wing, i.e., the blades are no longer hydrodynamic.
Accordingly, there is a need for a mill having fully stable, yet hydrodynamic folding blades that do not require a mechanical advantage or mechanism to operate.